Mulling is the process of grinding up a sample into fine powder through mortar and pestle that is dispersed in a paraffin for infrared spectroscopy.
Applying another flat surface and sandwiching the paste under gentle compression renders the sample ready for analysis.
[1] There are a variety of mineral oils used as mulling agents, their relevant differences being the absorption bands in their respective infrared spectra.
Another mulling agent that is commonly used is Fluorolube, essentially a fluorocarbon-based solution that exhibits strong carbon-to-fluorine bond absorptions from 1300 cm−1 onwards to 400 cm−1 in the mid-infrared spectrum.
This allows for all of the spectral features of the sample to be seen in an infrared spectrum, since the regions masked by each specific mulling agent are unaffected by the other.